1. Field of the Invention
The present invention relates to apparatus for determining the pore volume distribution of a powdered or porous solid sample by non-wetting liquid intrusion, and more particularly to a mercury porosimeter that provides for determining the pore volume distribution of two or more samples simultaneously by intrusion and extrusion of a non-wetting liquid such as mercury under ascending and descending pressures between ambient to 60,000 psi for pore size analysis from approximately 5 microns to 0.0036 microns.
2. Description of Related Art
Many materials, both natural and man-made, contain pores that range in depth and diameter from a few Angstroms to a few millimeters. Measuring the smallest pores, called micropores, is done most accurately by gas sorption. Mesopores, which are intermediate size pores, are commonly measured by either gas sorption or high-pressure mercury intrusion. Larger pores, called macropores, and the voids between closely packed or compressed particles, are best suited for a method of low-pressure mercury intrusion.
This invention relates to the measurement of mesopores and small macropores with the use of high-pressure mercury intrusion.
Mercury porosimeters operate upon a physical principle that a non-reactive, non-wetting liquid will not penetrate fine pores until sufficient pressure is applied to force its entry. The relationship between the applied pressure and the pore diameter into which mercury will intrude is given by the Washburn equation: EQU P=(-4 .gamma. cos .theta.).div.D
where P is the applied pressure, D is the pore diameter, .gamma. is the surface tension of mercury, 480 dynes per CM.sup.-1, and .THETA. is the contact angle between mercury and the pore wall, usually taken as 140.degree.. The mercury porosimeter monitors the volume of mercury intruded into and extruded from the sample as a function of pressure, which permits analytically generating pore size and volume distributions from the Washburn equation. When dealing with mesopores and macropores, high-pressure mercury intrusion and extrusion is utilized. A porosimeter must be able to subject mercury to pressures ranging between ambient to 60,000 psi which allows for pore size analysis from around 5 microns to 0.0036 microns pore diameter.
U.S. Pat. No. 4,170,129 (the '129 patent) discloses a method for determining the pore volume distribution of a powder sample by mercury intrusion, and measurement over a continuously ascending pressure. The '129 patent establishes the heretofore preferred method of determining the pore volume distribution of a powder sample by mercury intrusion.
As discussed in the '129 patent, the pore volume distribution of a powder or porous solid sample is desired for manufacturing of various materials such as filters, adsorbents, catalysts, porous rock separators, and the like. In certain of these materials such as used for metal and ceramic parts, structural failure can be anticipated if they have excessive pores. Alternately, materials used for catalysts should have open and accessible pores.
A conventional mercury porosimeter is comprised of a rigid housing having a high pressure cavity that can withstand internal pressures of sixty thousand pounds per square inch (psi). A penetrometer containing a test sample is mounted in the high pressure cavity. The penetrometer, including its capillary, is filled with mercury. A portion of the housing high pressure cavity that contains the penetrometer capillary includes typically a metal sheath that functions as one plate of an electrical capacitor. The penetrometer also includes an electrode that is in contact with the mercury contained within the capillary which also functions as the other plate of an electrical capacitor. As pressure is applied, the column of mercury recedes in the capillary changing the capacitance between the two plates. A mathematical relationship between the electrical capacitance measured and the volume of mercury is clearly established which is used in conjunction with internal pressure to obtain the proper mercury intrusion and extrusions curves for measuring pore volume distribution.
Finally, a plot of the measured data shows the relationship between the mercury intruded, the applied pressure, and the pore diameter.
In applications utilizing the hereinabove described techniques for determining pore size, multiple sample measurements are often desired for various reasons. To verify measurements and accuracies, dual test samples are often taken and run back to back. However, running back to back samples essentially doubles the time requirement for each test sequence. Each test requires placing a single test penetrometer containing the sample and a quantity of mercury into the apparatus. The sample in the apparatus must then be pressurized from ambient up to approximately 60,000 psi and back to ambient while making and recording sensitive capacitive measurements.
In a conventional high-pressure mercury porosimeter, because of the extremely high pressures involved, access to the high-pressure cavity is done through a single threaded port that accesses essentially a single cavity that tests individual samples. Each time the high-pressure porosimeter is used, a single sample is taken and the machine must be sealed and closed tightly, ascending and descending pressures applied while the sample readings are taken, and then the pressure withdrawn to remove the sample. Taking a series of individual test samples in serial relationship is, thus, a time consuming process, especially if the user has a large volume of different samples that must be analyzed.
The present invention overcomes the problems of the prior art by providing a high-pressure mercury porosimeter that has a single access port that contains a cavity that includes two or more penetrometers, each having a capillary so that two independent samples can be tested under high pressure, up to 60,000 psi, that requires substantially the same amount of time between start-up and shut-down as previously experienced using a single sample. This greatly increases the efficiency of the porosimeter and greatly reduces the per sample analysis time. The present invention accomplishes this without greatly increasing the internal volume of the porosimeter, which would adversely affect the ability to attain the high pressures involved (60,000 psi). The larger the volume of the cavity, the more oil is needed to create the high pressures required resulting in greater compressibility factors which can physically limit present day equipment from obtaining 60,000 psi. Using the present invention, two or more independent separate powder or porous solid samples can be obtained simultaneously, up to 60,000 psi, for pore size analysis of samples ranging from 5 microns to 0.0036 microns, by requiring only a single port access to the high-pressure housing and cavity.